1. Field of the Invention
The invention relates to an elastomeric article used in the form of a pre-stretched tubular member supported by a removable core. The articles may be used for the splicing and termination of high voltage power cables as well as sleeves for sealing pipes.
2. Descriotion Of The Prior Art
A pre-stretched tube (PST) is a tubular member which is supported in a stretched condition on a support member, usually in the form of an easily removable core. The core of the PST can be external, i.e., on the outside of the tubular member, or it can be inside the tubular member, such as is taught in U.S. Pat. No. 3,515,798, incorporated herein by reference. Preferably, the core is internal and is a one-piece rigid spiral core having interconnected adjacent coils in a closed helix configuration, as taught in U.S. Pat. No. 3,515,798.
The extent of stretching of PST is related to the hardness of the elastomeric material which is quantified by Shore A hardness values. The current PST materials generally have Shore A hardness values ranging from about 43 to about 60 and are generally limited to a maximum of about a 3 to 1 expansion ratio. Expansion over about 3:1 usually resulted in the tube splitting or tearing. Consequently, the diameter of the stretched elastomeric tube is limited to about three times the size of the tube in its unstretched condition. Further limitations are imposed by the ability of the core material to support the stretched tube without collapsing prematurely. The more the PST is expanded, the greater is the force exerted by the tube as it tries to recover. Consequently, the core must be designed to withstand this force. Where the core is designed for easy removability, there is a trade-off between its wall thickness and its ability to collapse on demand. To withstand higher forces, the wall thickness of the core must be increased. This adversely affects removability and cost of the core.
Current practice in the industry utilizes an inner plastic core which pre-stretches the tubing such that electrical cables may be inserted therein. Pre-stretched tubes on removable cores may be used in the repair of electrical cables. They are often used to protect spliced connections such that the spliced cable is threaded through a PST carrying support core which has an internal diameter larger than the outside diameter of the cable. With the PST positioned over the cable splice, the plastic core is removed allowing the stretched elastomeric material to recover, approaching its original size, so that the tubing fits snugly over the repaired section of cable.
The range of sizes, i.e., external diameters of spliced cable which may be protected by pre-stretched tubing, is limited. This limitation results from the balance which must be maintained between the recovery force exerted by the stretched elastomeric tube and the ability of the supporting core to resist crushing. The range of cable diameters to which PSTs may be applied could be extended by use of a relatively soft PST with a high expansion ratio. Therefore, a need exists for a softer elastomer, suitable for use in pre-stretched tubes, which possesses an expansion ratio larger than currently available materials. This higher ratio tubing could provide a PST for larger diameter cable and multilayer constructions suitable for high voltage applications.
The use of PST in high voltage applications has increased in recent years. Current PSTs are effective up to voltage ratings of about 600 volts. There is a need to increase this rating to as high as 5 kilovolts in high voltage applications. Also, there is a need to achieve these high voltage ratings using relatively soft rubbers such as ethylene propylene diene monomer rubber (EPDM). The dielectric strength of a rubber is also important and it is desirable to have a dielectric strength above about 450 V/mil in high voltage applications. The higher the dielectric strength of the rubber, the thinner the PST wall can be, which results in less material used and less stress on the core.
It is, therefore, an object of this invention to provide pre-stretched tubes comprised of low hardness, high expansion ratio elastomeric rubbers, supported on removable plastic cores. Another object of this invention is to provide elastomer PST compositions for use in high voltage applications.